1. Field of the Invention
The present invention generally concerns the acoustic measurement of fluid flow velocity, including in-vivo measurement of blood flow in animals.
The present invention particularly concerns a modification to the selective, regional, electro/acoustic or acousto/electric gain of at least one acoustic transducer of an acoustic flowmeter of either the transit time or Doppler types. The regionally variable gain compensates for differing acoustic attenuations that are presented to different portions of an acoustic path proceeding to and from the transducer(s) through both (i) the fluid for which dynamic flow is measured and (ii) material surrounding the flowing fluid.
2. Description of the Prior Art
2.1 Desirable Properties of an Acoustic Flowmeter, Particularly for Blood Flow Analysis
An acoustic flowmeter would preferably permit direct measurement of the rate of fluid flow regardless of the dimensions of a conduit, or lumen, within which the fluid flows. An implantable acoustic flowmeter for blood flow measurement would preferably permit direct measurement of the rate of blood flow regardless of vessel dimensions or any change in dimensions during the period of measurement.
An acoustic flowmeter would preferably be accurate. An implantable blood flowmeter would exhibit accuracy over the period of a long-term in-vivo implant.
An acoustic flowmeter would preferably permit direct measurement of the rate and the volume of fluid flow regardless of the profile of the velocity of the flowing fluid, e.g. turbulent flow or stratified flow, within the conduit of lumen. An acoustic flowmeter for blood flow measurement would preferably permit direct measurement of the rate and the volume of blood flow regardless of the stratified velocity profile of blood flowing within a blood vessel.
An acoustic flowmeter would desirably be insensitive to the angular alignment between a lumen or conduit and a flow sensor. An acoustic flowmeter for blood flow measurement would preferably be insensitive to the angular alignment between a blood vessel and a flow sensor axis.
An implantable acoustic flowmeter for blood flow measurement would preferably be operative with a non-constrictive flow probe, useful on various sizes of vessels, pulsating arteries, veins, growing vessels or even groups of vessels.
2.2 Exemplary Acoustic Flowmeters for Blood Flow Analysis
An early ultrasonic blood flowmeter was designed and built in the USA by Franklin, Baker, Ellis, and Rushner, in 1959. It employed a transit-time technique to measure fluid flow velocity. Transit time acoustic flow measurement is now widely used to measure liquid flow in industry but has not been widely used until recently for in-vivo measurement of blood flow.
U.S. Pat. Nos. 3,575,050 and 3,906,791 to Lynnworth describe a number of transit time flowmeters and transducers. Lynnworth's preferred transducers were piezoelectric elements that (i) subtended the full width of a lumen, or blood vessel, and (ii) uniformly acoustically illuminated the entire cross section of fluid flow. Lynnworth placed his transducers both at the two ends of simple acoustic paths directed obliquely across the lumen, and at the two ends of longer acoustic paths that underwent several reflections. Industrial acoustic transit time flowmeters based on the Lynnworth patents are still being manufactured, in 1992, by Panametrics, Inc., of Waltham, Mass., U.S.A.
Rader later alleged that when the full cross-sectional area of a blood vessel was acoustically illuminated with a constant level of ultrasound then the volume of blood flow within the vessel could be measured with a transit-time system independently of the vessel area. Reference R. D. Rader. "A Diameter-Independent Blood Flow Measurement Technique", Medical Instrumentation, Vol 10 pp. 185-188 (1976). Rader's electronic implementation, however, suffered from an unstable zero-flow offset.
Some time later Drost wrote about the physical principles of a transit-time volume flowmeter, and developed certain electronic circuitry. Reference C. J. Drost, "Vessel Diameter-Independent Volume Flow Measurements Using Ultrasound", Proc. San Diego Biomed. Symp. San Diego, Calif.: Vol. 17, pp. 299-302 (1978).
This flowmeter became the subject of U.S. Pat. Nos. 4,227,407 for "Volume Flow Measurement System" and 4,391,124 for an Electroacoustic Transducer Calibration Method and Apparatus", assigned to Cornell Research Foundation, Inc. (1980).
Under support of the National Institute of Health (NIH), the Drost flowmeter was put through a program of in-vivo use and validation by physiologists at Cornell University, Ithaca, N.Y. Linearity, long-term stability, and a low and stable zero-offset were stated to have been obtained. A commercial version of this flowmeter is now marketed by Transonic Systems, Inc. as Transonic Model T101.
2.3 General Theory of the Operation of a Prior Art Transit-Time Flowmeter
The discussion, and the referenced drawings, of prior art acoustic flowmeters contained within this section is partly derived from the article "Ultrasonic Flowmeter Uses Wide-Beam Transit-Time Technique" by R. G. Burton and Dr. R. C. Gorewit appearing in Medical Electronics, Vol. 86, No. 2, at pages 68-73 (1984).
Early, conventional, transit-time flowmeters illuminated only a segment of the flow, sensing the flow velocity in that segment. The relatively more recent wide-beam transit-time flowmeters evenly illuminate the full cross-section of the vessel. Reference, for example, the double transducer, direct path, configuration of a prior art transit time flowmeter shown in FIG. 1. The transit-time of the ultrasound is then independent of the vessel dimensions. It is supposed to permit the sensing of the volume, as well as the velocity, of the flow directly. Reference Drost, op cit.
The signal of the flow probe shown in FIG. 1 is a function of the angle between sound beam and flow axis; it is thus sensitive to blood vessel misalignment. This problem is largely solved in the prior art modified transit-time double-transducer reflected-path flowmeter diagrammed in FIG. 2. In this prior art flowmeter configuration the fluid flow path is traversed twice along a reflective acoustic pathway. If the lumen is inclined relative to the transducer(s) than one acoustic path through the lumen will be at a relatively steeper angle over a relatively shorter distance while the second, reflected, portion of the path will be at a relatively shallower angle over a relatively longer distance. This geometry results in a first order correction to any signal error due to misalignment between the flow, or lumen, axis and the axis of the transducer sensor. The reflector, in the form of an L-shaped bracket, also serves to hold the vessel within the acoustic path.
Although Doppler acoustic flowmeters operate on a different principle then do transit time flowmeters, a Doppler flowmeter also exhibits sensitivity to misalignment. The transducer configuration of a prior art single-transducer Doppler acoustic flowmeter is diagrammed in FIG. 3, and the transducer configuration of a prior art dual-transducer Doppler acoustic flowmeter is diagrammed in FIG. 4.
An previous electrical instrumentation scheme for a transit time acoustic flowmeter is shown in schematic block diagram in FIG. 5. In this scheme both upstream and downstream transit-time measurements are made alternately. The circuit uses a master oscillator (for a time base) and memory elements (to store transit-time information). The timing circuitry first energizes one of the transducers to emit an acoustic signal, after which time the other transducer is connected to the receiver. The received acoustic signal, containing the full flow information, is transduced into an electrical signal, amplified and fed into a circuit which measures the phase difference between the received signal and the master oscillator signal. This phase difference is indicative of the time of transit, and thus of the rate of fluid flow. The average phase-shifted received signal is used to update one of two memory elements.
After waiting to let all acoustic echoes die out, the roles of transmitting and receiving transducer are reversed for a measurement of transit-time in the opposite upstream/downstream direction. The resulting phase shift is again stored, now in the other memory element. The difference between the two stored values is representative of the difference in acoustic propagation upstream and downstream in the flowing fluid, and thus of the rate of fluid flow. This sequence of measurements is repeated at intervals shorter than the fluid can appreciably change velocity, typically every few milliseconds.
An electronic zero-flow reference signal is generated by subtracting two consecutive upstream phase measurements, rather than an upstream and a downstream measurement. This eliminates the need for clamping the vessel to establish a zero-flow baseline. A crystal-controlled internal factor reference transit-time delay to read out directly in ml/min.
2.4 The Particular Theory of Operation of a Prior Art Transit-Time Flowmeter
The transit-time of a sound wave traveling between two transducers is a function of the velocity of the sound-conducting medium times the acoustic path length. This established method of measuring flow velocity in liquids and gases was implemented for biomedical use by Plass, among others. Reference Plass, K.G.; A new ultrasonic flowmeter for intravascular application. IEEE Trans. Bio-Med. Eng., vol. BME-11, pp. 154-156, Oct, 1964. Plass's intravascular system used the difference in transit time between an upstream and downstream projected sound burst to measure flow velocity with known zero reference. Rader, et al. used the same detections scheme for an extra-vascular flowmeter. Reference Rader, op cit. A noted drawback of the implementations of Plass and of Rader, et al. is the path-length-sensitivity of the transit-time technique: an error in vessel inside diameter estimation produces a proportional error in volume flow output.
Drost alleged that the diameter sensitivity of the acoustic transit-time measuring technique could be turned into a major advantage, providing direct volume flow metering capability. Drost alleged that his detection scheme and probe design produced true volumetric output for a wide range of vessel diameters with one size of flow probe, independent of flow profile.
2.5 Prior Art Probe Design for a Transit-Time Flowmeter
Drost describes in the aforementioned U.S. Pat. Nos. 4,227,407 and 4,391,124 that a rectangular beam of ultrasound, uniform in intensity across its height (dimension h in FIG. 1) is essential to producing a flowmeter output that is indicative not only of flow rate, but also of flow volume. Producing such a field is an empirical procedure. The acoustic field intensity of finite-sized ceramic transducers fall off towards the transducer edges. Lynnworth, who appreciated the utility of uniform illumination of a lumen even before Drost, recognized this small non-uniformity in transducers. In Lynnworth's U.S. Pat. No. 3,906,791 for an "Area Averaging Ultrasonic Flowmeter" he discusses "shading" the transducer electrodes to compensate for the weakened acoustic field at the edges of the acoustic beam emitted by the transducer. Reference also the earlier patent of Lynnworth U.S. Pat. No. 3,575,050 for a "Fluid Flowmeter".
The prior art, as exemplified by both Lynnworth and Drost, shows and describes that the acoustic illumination of a lumen for the transit time measurement of fluid flow rate and also, particularly, of fluid flow volume, should be uniform. The present invention will be seen to teach, and function, contra. One problem with uniform acoustic illumination plagues both transit time and Doppler acoustic flowmeters, and flow probes, in any environment, as is common, where material located proximately to, or surrounding, the flowing fluid within the lumen attenuates sound differently than does the fluid itself. The existence of such material is all but inevitable in, for example, in-vivo acoustic blood flow measurement where a blood vessel is surrounded by fat and/or connective tissue that attenuates sound (or ultrasound) much differently than does blood.
It does not matter that this incidentally- and acoustically-illuminated material is not moving. It affects the received acoustic signal in a manner that induces inaccuracy in the measured fluid flow rate, or volume. Namely, the material is not equally abundant (or sparse) at all regions of the acoustic paths. A transducer-to-transducer acoustic path directly through the center of the lumen, or blood vessel, likely couples very little of the differentially-attenuating material external to the lumen, or blood vessel. Portions of the transducer-to-transducer acoustic path proceeding through the side regions of the lumen, or blood vessel, are opposite. These portions of the acoustic path typically couple a good deal of the differentially-attenuating material that is external to the lumen, or blood vessel. In fact, it may be imagined that those portions of the acoustic path that just graze the regions of the lumen, or blood vessel, proximate to its wall proceed almost entirely through the material external to the lumen, or blood vessel, and only but a short ways through the flowing fluid, or blood.
The different attenuation provided to different portions of the acoustic path by the material surrounding the lumen, or blood vessel, serves to diminish those contributions to the flow signal resulting from fluid flow occurring within peripheral regions of the lumen relative to those contributions resulting from flow within the central region of the lumen. This means that flow within the peripheral regions of the lumen is not weighed as highly in the determination of average flow as flow occurring within the central region of the lumen (or, vice versa, that flow within the central region of the lumen is weighed more highly in the determination of average flow than is flow occurring within the peripheral regions of the lumen).
If regional flow within a lumen, or blood vessel, was everywhere the same, it might not matter if some regions were proportionately under, or over, represented in determination of an "average flow". Typically, however, the profile of flow velocity is stratified, with a slower velocity of fluid flow near the lumen walls where frictional resistance is experienced than at the lumen's center. Accordingly, such uneven weighting of flow from different regions of a lumen, or blood vessel, that is flowing fluid, or blood, at a stratified flow velocity profile as results from an even uniform acoustic illumination of the lumen and the attenuating material surrounding the lumen serves to inaccurately overestimate the measured average flow velocity, and flow volume. This effect is called "attenuation-based skew", and causes the average flow velocity to be overstated.
It will be further discussed in the aforementioned companion patent application for a VARIABLE ACOUSTIC ILLUMINATION OF A LUMEN FLOWING FLUID AT A STRATIFIED FLOW VELOCITY PROFILE IN ORDER TO MORE ACCURATELY DERIVE TRUE AVERAGE FLOW, PARTICULARLY DURING DOPPLER ACOUSTIC BLOOD FLOW MEASUREMENT that a Doppler acoustic flowmeter suffers from yet another type of error resulting from uniform acoustic illumination of (but a portion of) a lumen, or blood vessel, flowing fluid at a stratified profile of flow velocity. The measured average flow is again overstated. Both applications show that the utility of uniform illumination in acoustic fluid flow measurement must be reconsidered.
Normally the accuracy of an instrument such as a flowmeter is established by calibration against formerly-existing instruments, or against standards. The considerable inaccuracies of existing acoustic flowmeters in in-vivo blood flow measurement may have persisted because it has been difficult to calibrate such flowmeters in their actual environment of use. In other words, the simple expedient of observing how much fluid is accumulated in a reservoir after a set time in order to calibrate the measured flow is not available when the flow is that of blood in the closed system of a live animal.